A global treaty to fight climate change will be postponed by at least six months and possibly a year or more, senior negotiators and politicians conceded today.

In a day of gloomy statements, the world's key industrialised nations said they had abandoned hope of a legally binding treaty at the Copenhagen summit next month and had begun to plan only for a meeting of world leaders.

The stark statements follow weeks of pessimism and represent a significant downgrading of the summit's goal.

In London, Ed Miliband, the UK climate change secretary, became the first British politician to acknowledge publicly that Copenhagen would produce no legal climate change treaty.

Speaking in the House of Commons, he said: "The UN negotiations are moving too slowly and not going well." He went on to describe a "history of mistrust" between developed and developing nations with negotiators "stuck in entrenched positions", an impasse that prompted African nations to stage a walkout at the negotiations this week.

In Barcelona, where last-ditch negotiations are taking place, it became clear today the best hope for Copenhagen is a "politically binding" agreement, which rich countries hope will have all the key elements of the final deal, including specific targets and timetables for greenhouse gas emissions cuts and money for poor countries to cope with climate change.

A British government official said: "It would be substantive. It would set timelines, and provide the figures by which rich countries would reduce emissions, as well as the money that would be made available to developing countries to adapt to climate change."

But, she said, a legally binding agreement "could take six months, up to a year, but we would want it to be [signed] as soon as possible."

Sources said a meeting in Mexico in December 2010 would be more likely to see the legal treaty sealed.

The news of the delay was met with resignation by developing countries and NGOs. "Politically binding agreements are worth very little," said Lumumba Di-Aping, chair of the G77 group of developing countries. "Tell me of any politician who delivers a politically binding agreement."

The delay was said to be caused by a combination of time running out in the increasingly rancorous UN negotiations and the inability of the US – the world's biggest cumulative emitter – to commit to specific targets and timetables by passing a domestic law.

The Obama administration made clear on Wednesday it thought a legal treaty was impossible in Copenhagen. Today it further inflamed opposition to its Senate bill when Barbara Boxer, chairman of the environment committee, defied a Republican boycott to vote through a sweeping plan to reduce greenhouse gas emissions 20% over 2005 levels by 2020.

The UN secretary general, Ban Ki Moon, said on Tuesday a delay of a year would be too long, while developing countries were dismayed tonight that they had not been formally told of the delay. "We cannot afford delaying tactics in any way. It's a matter of life and death," said Makase Nyaphisi, the Lesothan ambassador speaking on behalf of the UN's least developed group of 49 countries.

Speaking in Barcelona, Artur Runge-Metzger, the European commission's chief negotiator, said: "It is a Catch-22 situation. People are waiting for each other so it is difficult to blame anyone. [But] the US position is significant. Clearly the US has been slowing things down."

Both Miliband and the prime minister, Gordon Brown, are to attend Copenhagen, with Brown calling it the last chance to prevent "catastrophic" climate change.

Brown, President Lula of Brazil, President Sarkozy of France and other heads of state have already said they will go.

It is now more likely that President Obama will go because he will not be forced to sign a legally binding agreement which the US Senate could reject.

Miliband's comments were the first public reappraisal of the British position since officials began to shift the line following downbeat comments last week from the Copenhagen host, Danish prime minister Lars Lokke Rasmussen.

Government sources said it had become increasingly obvious amid slow negotiations that a legally binding treaty in December was unlikely.

But one insisted that political commitments would move to legal ones, pointing out that the Kyoto protocol followed the same course from political to legal agreement. "I don't think we are downbeat about this," said one.

They also said pledges made at Copenhagen would be as difficult to escape as if they were legally binding, because nations would have made their commitments at the very public forum of a UN meeting.